Accessing a Database in Java

By:

Hafez Seleim

Data Storage

• Memory (Small (1 - 16 GB), Non-persistent)

• Text Files (inefficient, difficult to organize)

• Databases

– Manages possibly huge quantities of data

– Facilitates fast and easy access

– Makes data integrity guarantees

– Implementations (MySQL, Microsoft SQL Server, , Oracle, )

Database Organization

• A single database has multiple tables

• A table has multiple rows

• Each row has multiple columns

• Each column represents a different data category

Table of actors:

Database Operations (CRUD)

• Create data in a table

• Read data from a table

• Update data in a table

• Delete data in a table • Structured Query Language (SQL) : language used to interface with a

database, allows a user to perform CRUD operations on a particular database

Tables create table Courses ( courseId char(5), subjectId char(4) not null, courseNumber integer, title varchar(50) not null, numOfCredits integer, primary key (courseId) );

create table Students ( ssn char(9), firstName varchar(25), lastName varchar(25), birthDate date, phone char(11), primary key (ssn) );

create table Enrollment (

ssn char(9),

courseId char(5),

dateRegistered date,

grade char(1),

primary key (ssn, courseId),

foreign key (ssn) references Students (ssn),

foreign key (courseId) references Courses

(courseId)

);

insert into Courses (courseId, subjectId, courseNumber, title, numOfCredits) values ('11113', 'CSCI', '3720', 'Database Systems', 3);

select * from Enrollment;

UPDATE Courses SET title = ‘DB’ WHERE title = ‘'Database Systems’;

• DELETE FROM Courses WHERE title = ‘DB’ ;

JDBC Overview

1.JDBC – Java Database Connectivity 2. Get a Connection to the database. 3. Create a Statement using the Connection. 4. Execute the Statement with SQL string. 5. Use the results.

JDBC Code static final String URL = "jdbc:mysql://dbserver/world";

static final String USER = "student";

static final String PASSWORD = "secret";

// 1. Get a Connection to the database. Connection connection =

DriverManager.getConnection( URL, USER, PASSWORD );

// 2. Create a Statement

Statement statement = connection.createStatement();

// 3. Execute the Statement with SQL command.

ResultSet rs = statement.executeQuery("SELECT * FROM ...");

// 4. Use the Result.

while ( rs.next( ) ) {

String name = rs.getString("name");

Database URL

String DB_URL = "jdbc:mysql://dbserver:3306/world";

The format of a database URL is:

Protocol Sub-protocol Hostname Port DatabaseName

Port is the TCP port number where the database server

is listening.

3306 is the default port for MySQL

Use hostname "localhost" for the local machine.

JDBC Overview

creates Statements

for database actions

selects a specific Connection type

and instantiates it

Where is the Database Driver?

Driver is in a JAR file.

JAR file must be on the CLASSPATH.

Use one of these:

1. add as an external jar file to your IDE project

2. add the JAR to your CLASSPATH CLASSPATH = /my/path/mysql-connector.jar;.

3. add JAR using the Java command line: java -cp /my/path/mysql-connector.jar ...

4. Put JAR file in the JRE/lib/ext directory: C:/java/jre1.6.0/lib/ext/mysql-connector.jar

How to Execute SQL Commands

The Statement interface defines many execute :

Resultset rs =

statement.executeQuery("SELECT ...");

use for statements that return data values (SELECT)

int count =

statement.executeUpdate("UPDATE ...");

use for INSERT, UPDATE, and DELETE

boolean b =

statement.execute("DROP TABLE test");

use to execute any SQL statement(s)

Object-Relational Mapping

Purpose

• save object as a row in a database table

• create object using data from table

• save and create associations between objects

Design Goals

• separate O-R mapping service from our application

• localize the impact of change in database

Goal

• Applications need to save data to persistent storage.

• Persistent storage can be database, directory service, XML files, spreadsheet, ...

• For O-O programming, we'd like to save and retrieve objects to/from storage.

How to do Object Persistence

Choices for How to do Object Persistence?

1.write your own DAO using JDBC

2.Use an Object-Relational Mapping (ORM) Framework

– Hibernate, TopLink, MyBatis, Apache Cayenne

3.Use a Standard Persistence API.

– Java Persistence Architecture (JPA)

• standard used in JavaEE

• implemented by EclipseLink, Hibernate, OpenJPA

The Problem with Databases

Object-Relational Paradigm Mismatch

• Database stores data as rows in tables, which are not like objects.

• Objects have associations and collections databases have relations between tables.

• Objects are unique, database row data is copied each time you query it.

Mapping an Object

LOCATIONS

id name address

101 Kasetsart University 90 Pahonyotin ...

102 Seacon Square 120 Srinakarin ...

ku : Location

id = 101

name = "Kasetsart University"

address = "90 Pahonyotin ..."

object diagram

save( )

4 Approaches to ORM 1. No ORM -- JDBC in my code.

No Layers! Put the JDBC right in your app code.

2. Do It Myself.

Write your own DAO using JDBC.

3. Use a Framework.

Hibernate, MyBatis, TopLink, or other.

4. Use a Standard.

Java Persistence Architecture (JPA) or Java Data Objects (JDO) provide a standard API that have many implementations.

Bean and DAO public class StudentBean { private Integer age; private String name; private Integer id; public void setAge(Integer age) { this.age = age;} public Integer getAge() { return age;} public void setName(String name) { this.name = name;} public String getName() { return name;} public void setId(Integer id) { this.id = id;} public Integer getId() { return id; } }

public class StudentDAO { public void create(String name, Integer age) { String SQL = "insert into Student (name, age) values (?, ?)“; // send the command to the database System.out.println("Created Record Name = " + name + " Age = " + age); } public Student getStudent(Integer id) { String SQL = "select * from Student where id = ?"; //get the data and fill it in new object return student; } public List<Student> listStudents() { String SQL = "select * from Student"; return students; } public void delete(Integer id) { String SQL = "delete from Student where id = ?"; } public void update(Integer id, Integer age){ String SQL = "update Student set age = ? where id = ?"; System.out.println("Updated Record with ID = " + id ); } }

Persistence Frameworks

Hibernate - most popular open-source persistence framework for Java. NHibernate for .Net.

Uses POJOs and object-query language. Completely decouple Java from database. Can reverse engineer.

MyBatis - simple, uses SQL maps. Database schema not transparent to Java code.

Cayenne - Apache project, has GUI modeler that eliminates need to write xml. Can reverse engineer database or generate database schema & Java code.

TopLink (Oracle)

Torque (Apache DB)